1. Field of the Invention
This invention relates to a magnetic recording medium on which a magnetization pattern is formed by magnetic transfer.
2. Description of the Related Art
With an increase in information quantity, there has been a demand for a magnetic recording medium which is high in memory capacity, low in cost and preferably requires a short time to read out a necessary part of data (a magnetic recording medium which allows so-called high-speed access). As an example of such a magnetic recording medium, there has been known a high recording density magnetic medium such as a hard disc or a ZIP (Iomega) in the form of a flexible disc. In such a high recording density magnetic medium, the recording area is formed by narrow data tracks. In order to cause a magnetic head to accurately trace such narrow data tracks and reproduce the data at a high S/N ratio, the so-called servo tracking technique has been employed.
In order to perform the servo tracking, it is necessary to write servo information such as servo tracking signals for positioning the data tracks, address signals for the data tracks and reproduction clock signals on the magnetic recording medium as a preformat upon production thereof. At present, such preformat recording is performed by the use of a specialized servo recording apparatus (a servo track writer) However, the preformat recording by the conventional servo recording apparatus is disadvantageous in that it takes a long time since the servo information must be recorded on the magnetic recording medium one by one by the use of a magnetic head, which deteriorates the productivity.
As a method of recording the preformat accurately and efficiently, there has been proposed, for instance, in Japanese Unexamined Patent Publication No. 63(1988)-183623, and U.S. Pat. No. 6,347,016, a magnetic transfer method in which a pattern which is formed on a master information carrier and represents servo information is copied to a magnetic recording medium (a slave medium) by magnetic transfer.
In the magnetic transfer, the magnetization pattern representing the information (e.g., servo information) carried by a master information carrier is magnetically transferred from the master information carrier to a slave medium by applying a transfer magnetic field to the slave medium and the master information in close contact with each other, and accordingly, the information carried by the master information carrier can be statically recorded on the slave medium with the relative position between the master information carrier and the slave medium kept constant. Thus, according to the magnetic transfer, the preformat recording can be performed accurately and the time required for the preformat recording is very short.
U.S. Pat. No. 6,347,016 discloses a magnetic transfer method using a patterned master having thereon an irregularity pattern (a pattern of protruding portions and recessed portions) representing information to be transferred. Further, we have proposed, in our Japanese Unexamined Patent Publication No. 2001-14467, a magnetic transfer method in which a soft magnetic layer small in coercive force is formed on the surface of the protruding portions of the substrate of the master information carrier, the magnetic layer of the slave medium is DC-magnetized in one direction of the recording tracks and a transfer magnetic layer is applied to the slave medium in the direction opposite to the direction of the DC-magnetization with the magnetic layer of the slave medium held in close contact with the soft magnetic layer of the master information carrier.
In order to realize a high density recording, it is necessary to reduce the particle volume of the magnetic material. However, as the particle volume of the magnetic material becomes smaller, it becomes impossible for a magnetic head to record a high frequency signal on a recording medium in saturation recording.
Though a data signal can be detected by signal processing such as PRML even if it is not recorded in saturation recording, there is a strong probability that a servo signal becomes undetectable if it is not recorded in saturation recording.
In view of the foregoing observations and description, the primary object of the present invention is to provide an inexpensive magnetic recording medium by recording a signal in saturation recording on a magnetic recording medium.
Another object of the present invention is to provide an inexpensive magnetic recording medium for distribution by recording not only a servo signal but a data signal on a magnetic recording medium.
In accordance with the present invention, there is provided a flexible magnetic recording medium comprising a non-magnetic layer and a magnetic layer consisting of ferromagnetic powder dispersed in binder which are formed on a base sheet in this order, wherein the improvement comprises that
the ferromagnetic powder is not smaller than 158 kA/m (about 2000 Oe) in coercive force Hc(VSM) as measured by a vibrating sample magnetometer and is not larger than 100 in KuV/kT (wherein Ku represents an anisotropy constant, V represents a volume, k represents a Boltzmann constant and T represents an absolute temperature) which is a parameter of thermal fluctuation, and
a magnetization pattern representing predetermined information has been formed on the magnetic layer by magnetic transfer.
It is preferred that the ferromagnetic powder be ferromagnetic metal powder or ferromagnetic hexagonal ferrite powder.
When the ferromagnetic powder is not smaller than 278 kA/m in coercive force Hc(VSM) as measured by a vibrating sample magnetometer, the magnetic recording medium is especially suitable as a read-only recording medium.
The coercive force Hc as measured by a vibrating sample magnetometer means a value obtained through measurement near the value for an observed time of 10 seconds.
It is preferred that the magnetic layer be not larger than 3 nm in center plane mean roughness.
Further it is preferred that the magnetic layer be not larger than 5xc3x9710xe2x88x922Txc2x7xcexcm in xcfx86m (magnetic flux densityxc3x97thickness of the magnetic layer).
Further it is preferred that the ferromagnetic powder be not larger than 1xc3x9710xe2x88x9217 cm3 in volume.
Further, it is preferred that the ferromagnetic powder be not smaller than 1xc3x97104 J/m3 in anisotropy constant Ku.
The xe2x80x9cnon-magnetic layerxe2x80x9d need not be completely non-magnetic so long as it is substantially non-magnetic and the magnetism thereof is sufficiently weak as compared with the magnetic layer consisting of ferromagnetic powder dispersed in binder.
It is preferred that the magnetization pattern representing predetermined information be formed on the magnetic layer by magnetic transfer in which a transfer magnetic field is applied to the magnetic recording medium in close contact with a master information carrier provided thereon a magnetic layer pattern representing the predetermined information so that a magnetization pattern corresponding to the magnetic layer pattern is formed on the magnetic layer of the magnetic recording medium. The xe2x80x9cmagnetic layer pattern representing the predetermined informationxe2x80x9d may comprise, for instance, a substrate having an irregularity pattern formed on the surface thereof and magnetic layers formed at least on protruding portions of the irregularity pattern on the substrate, a substrate having an irregularity pattern formed on the surface thereof and magnetic layers embedded in recessed portions of the irregularity pattern on the substrate, and a flat substrate and magnetic layers formed in a pattern on the flat substrate. That is, the master information carrier is so-called a patterned master information carrier which bears thereon information not as a magnetization pattern but as a magnetic layer pattern. As the magnetic layers of the master information carrier, soft magnetic layers are optimal.
The xe2x80x9cpredetermined informationxe2x80x9d may be, for instance, a servo signal.
When the ferromagnetic powder of the magnetic layer is not smaller than 158 kA/m (about 2000 Oe) in coercive force Hc(VSM) as measured by a vibrating sample magnetometer and is not larger than 100 in KuV/kT which is a parameter of thermal fluctuation, the effective coercive force of the magnetic layer at the signal frequency is increased so that saturation recording of signals by the normal magnetic head become impossible. Whereas, since recording is statically effected in magnetic transfer, signals can be recorded in saturation recording by magnetic transfer.
As disclosed in xe2x80x9cIEEE TRANS. ON MAG-17, No. 6, November 1981, pp3020 to 3020xe2x80x9d, magnetization-inversion-time-dependency of coercive force Hc is represented by the following formula.       Hc    ⁡          (      τ      )        =                    2        ⁢        Ku            Ms        xc3x97          [              1        -                                            kT              KuV                        xc3x97                          In              ⁡                              (                                                      A                    ⁢                                          xe2x80x83                                        ⁢                    τ                                    0.693                                )                                                        ]      
Since the coercive force Hc as measured by a vibrating sample magnetometer (VSM) is a value obtained through measurement near the value for an observed time of 10 seconds, the magnetization-inversion-time-dependency of coercive force Hc is standardized by Hc(VSM) as follows.       Hc    ⁡          (      τ      )        /      xe2x80x83    ⁢      "AutoLeftMatch"                  Hc        (                  xe2x80x83                ⁢        VSM        )            =              "AutoLeftMatch"                              [                          1              -                                                                    kT                    KuV                                    xc3x97                                      In                    ⁡                                          (                                                                        A                          ⁢                                                      xe2x80x83                                                    ⁢                          τ                                                0.693                                            )                                                                                            ]                    /                      [                          1              -                                                                    kT                    KuV                                    xc3x97                                      In                    ⁡                                          (                                                                        A                          ·                          10                                                0.693                                            )                                                                                            ]                              
wherein Ku represents an anisotropy constant, Ms represents saturation magnetization, k represents a Boltzmann constant (1.38xc3x9710xe2x88x9216 erg/K), T represents an absolute temperature, V represents a volume, A represents a spin precessional frequency (2xc3x97109/sec) and xcfx84 represents the magnetization inversion time.
For example, when the absolute temperature T=300K and KuV/kT=75, 100, 150 and 200, change of Hc(xcfx84)/Hc(VSM) with xcfx84 is as shown in the following table 1.
The table 1 is graphed as FIG. 3. As can be seen from FIG. 3, the effective coercive force increases as the xcfx84 becomes smaller. Further, for a given xcfx84, the effective coercive force increases as KuV/kT becomes smaller. Especially, when KuV/kT is not larger than 100, the magnetization-inversion-time-dependency of the effective coercive force Hc is large and the effective coercive force becomes not smaller than 1.5 times Hc(VSM) in the magnetization inversion time, 1xc3x9710xe2x88x927 to 1xc3x9710xe2x88x928, at the current signal frequency at which the normal magnetic head writes and erases. Accordingly, when the coercive force Hc(VSM) of the ferromagnetic powder of the magnetic layer is not smaller than 158 kA/m and KuV/kT is not larger than 100, the effective coercive force of the magnetic layer becomes too large for the normal magnetic head to write on the magnetic recording medium in saturation recording. That is, though saturation recording on the magnetic recording medium by the current magnetic head technology is impossible, signals can be recorded in saturation recording on the magnetic recording medium by magnetic transfer. Accordingly, a servo signal can be optimally recorded on the magnetic recording medium.
In order to record on the magnetic recording medium at a high density not lower than 1 Gbit/inch2, it is necessary that the magnetic material is fine in particle size, the magnetic layer has a smooth surface and the magnetic layer is small in thickness.
When the magnetic layer surface is rough, the S/N deteriorates due to spacing loss upon recording and/or modulation noise.
When xcfx86m (magnetic flux densityxc3x97thickness of the magnetic layer) is larger than 5xc3x9710xe2x88x922Txc2x7xcexcm, the S/N deteriorates due to output drop caused by self-demagnetization and/or waveform interference.
When the ferromagnetic powder is larger than 1xc3x9710xe2x88x9217 cm3 in volume, noise is increased and the S/N deteriorates.
When the ferromagnetic powder is smaller than 1xc3x97104 J/m3 in anisotropy constant Ku, recorded magnetization vanishes and the S/N deteriorates.